It is known that, in bipolar transistors, the linearity of the change (hereinafter, referred to as “capacitance characteristics”) in base-collector capacitance Cbc with respect to collector-emitter voltage Vce (or, base-collector voltage Vbc) may affect occurrence of harmonic distortion and EVM (error vector magnitude) during a high-output operation. In the case where a bipolar transistor has capacitance characteristics such that base-collector capacitance Cbc sharply increases in the voltage range in which base-collector voltage Vbc is positive (Vbc>0 V, which substantially corresponds to Vce<1.35 V), the following problems may arise in addition to the occurrence of harmonic distortion and EVM described above. That is, when a bipolar transistor is operated with a low power supply voltage Vce (Vce<1.35 V), the gain obtained during a high-frequency operation may be significantly reduced.
In order to address the above-described problems, there has been proposed a technique in which the doping concentration in a collector layer and the distribution of the doping concentration are controlled in order to improve the linearity of the capacitance characteristics of a bipolar transistor (e.g., see IEEE Transactions on Electron Devices, (United States), IEEE, January 2010, Vol. 57, No. 1, pp. 188-194). In IEEE Transactions on Electron Devices, (United States), IEEE, January 2010, Vol. 57, No. 1, pp. 188-194, the capacitance characteristics of the following eight heterojunction bipolar transistors (HBTs) illustrated in FIG. 10 were measured (see FIG. 11): two HBTs, namely, Type A and Type H, each including a collector layer constituted by one layer having a uniform doping concentration; and six HBTs, namely, Type B to Type G, each including a collector layer constituted by three sublayers in which doping concentrations were each independently controlled. FIG. 10 illustrates the doping concentration in the collector layer of each bipolar transistor prepared in the related art and the distribution of the doping concentration. FIG. 11 illustrates the capacitance characteristics of each bipolar transistor prepared in the related art.
In Type A and Type H, the collector layer was constituted by one layer having a uniform doping concentration. In this case, in Type A in which doping concentration was set relatively low, base-collector capacitance Cbc increased about 1.67 times as collector-emitter voltage Vce decreased from 6 V to 0 V as illustrated in FIG. 11. On the other hand, base-collector capacitance Cbc increased about 2.06 times in Type H in which doping concentration was set relatively high. In addition, capacitance Cbc was larger than that measured in Type A. Thus, Type A, in which the doping concentration in the collector layer was set lower, had better capacitance characteristics than Type H in which the doping concentration in the collector layer was set higher.
In Type B to Type G, the collector layer was constituted by three sublayers in which doping concentrations were each independently controlled. The middle sublayer of the three sublayers had a higher doping concentration than the other sublayers located on the respective sides of the middle sublayer. In this case, base-collector capacitance Cbc increased about, for example, 1.25 times in Type D as collector-emitter voltage Vce decreased from 6 V to 0 V as illustrated in FIG. 11. That is, the linearity of capacitance characteristics was improved compared with Type A and Type H, in which a one-layer structure was employed.